Conventionally, the programmable display device of the type mentioned is debugged generally by checking the eventual operation of the device as connected to the PLC to be used actually. On the other hand, it is also proposed to debug the display device by simulating functions of the PLC on a general-purpose personal computer without actually connecting the device to the PLC.
However, for the simulation of PLC functions on a personal computer, it is necessary to simulate all the functions of the PLC including its data communication function and ladder program for communication which are provided according to the specifications peculiar to the particular PLC. Moreover, PLCs are not universal in functions; those of different types have different functions even if provided by the same manufacturer. Thus difficulties are encountered in developing debugging systems per se.
Further such programmable display devices are generally so designed as to display the main menu on start-up, such that procedures presented on the main menu are thereafter selected one after another to perform the tasks required by the device and to process all the data needed by the operator.
The processing screen is often provided with a switch for a return to the preceding screen of information and a switch for proceeding to the next screen of information so that the current on-screen information can be moved one frame forward or reversely. Further another switch may be provided for a direct return to the main menu.
In recent years, however, on-screen information becomes complex, and many frames of on-screen information for data processing need to be implemented or passed before completely performing all the required data processing operations. Consequently, even if the on-screen information can be merely scrolled forward or reversely, the user is unable to recognize the position of the current screen of information in the entire quantity of on-screen information for processing, and also encounters difficulty in reaching the screen information required. Since the programmable display device of the type described is generally used at a site of work as in a plant, it is not infrequently required to urgently move the on-screen information to the position of the required frame.
To meet such requirement, it is possible to prepare pull-down or tabulated menus so as to enable the operator to recognize the overall relationship. Such forms of menus are useful for processing data in parallel manner, whereas if it is attempted to use menus in selecting different on-screen processing procedures in series according to options, many problems still remain to be overcome. For example, the on-screen information itself or the program becomes complex.